U.S. patent application number 10/253059 was filed with the patent office on 2003-04-10 for method for determining a time offset of a cdma signal.
Invention is credited to Schmidt, Kurt.
Application Number | 20030067938 10/253059 |
Document ID | / |
Family ID | 7701768 |
Filed Date | 2003-04-10 |
United States Patent
Application |
20030067938 |
Kind Code |
A1 |
Schmidt, Kurt |
April 10, 2003 |
Method for determining a time offset of a CDMA signal
Abstract
A method is described for determining a time offset of a CDMA
signal incurred during transfer. For each of a plurality of time
offset estimates, the received chip sequence is multiplied with a
conjugate complex scrambling code that is shifted by a number of
chips according to one of the time offset estimates; the chip
sequence is despreaded based on a spreading code of at least one
selected data channel to produce a symbol sequence, and an
estimated amplification factor is determined by summation of an
amount of the symbol sequence. The maximum of the amplification
factors for the time offset estimates when a selected data channel
is active, or the minimum of the amplification factors for the time
offset estimates when the selected data channel is not active is
then determined.
Inventors: |
Schmidt, Kurt; (Grafing,
DE) |
Correspondence
Address: |
DITTHAVONG + CARLSON, P.C.
SUITE A
10507 BRADDOCK ROAD
FAIRFAX
VA
22032
US
|
Family ID: |
7701768 |
Appl. No.: |
10/253059 |
Filed: |
September 25, 2002 |
Current U.S.
Class: |
370/441 ;
370/335; 375/E1.025 |
Current CPC
Class: |
H04J 13/0003
20130101 |
Class at
Publication: |
370/441 ;
370/335 |
International
Class: |
H04B 007/216 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 8, 2001 |
DE |
101 49 544.7 |
Claims
What is claimed is:
1. A method for determining a time offset of a CDMA signal
including data signal symbols borne on a plurality of data channels
and pilot signal symbols borne on a pilot channel, each of said
data channels being spread with a corresponding spreading code to a
chip sequence and subjected to amplification factors, wherein the
combined CDMA signal from the data channels and the pilot channel
is multiplied by a scrambling code, and wherein the CDMA signal has
undergone during transfer the time offset to be determined by a
number of chips, said method comprising: for each of a plurality of
time offset estimates, performing the steps of: multiplying a
received chip sequence with a conjugate complex scrambling code
that is shifted by a number of chips according to one of the time
offset estimates, despreading the chip sequence based on the
spreading code of at least one selected data channel to produce a
symbol sequence, and determining an estimated amplification factor
by summation of an amount of the symbol sequence; and determining a
maximum of the amplification factors for the time offset estimates
when a selected data channel is active, or determining a minimum of
the amplification factors for the time offset estimates when the
selected data channel is not active.
2. The method according to claim 1, wherein: the maximum and the
minimum of the amplification factors is determined for the time
offset estimates of the estimated factors when it is not known
whether the selected data channel is active or not.
3. The method according to claim 1, further comprising evaluating
the received chip series in a time window, in which a number of
chips is contained that corresponds to an integral multiple of the
length of the spreading code.
4. The method according to claim 1, wherein: the plurality of time
offset estimates includes each possible time offset within the
scrambling code.
5. The method according to claim 1, further comprising: storing
permutations of the product of the conjugate complex scrambling
code with the spreading code of the selected data channel that can
occur during despreading for each symbol in a permutation
matrix.
6. The method according to claim 5, wherein: the permutation matrix
is calculated at a beginning of a procedure for the received chip
sequence and that the permutation matrix is accessed during the
calculation of the estimated amplification factors.
7. The method according to claim 1, further comprising: selecting
the data channels that are either all active or all inactive,
wherein the maximum or the minimum is determined from the linear
combination of the amplification factors.
8. A computer-readable medium bearing instructions for determining
the time offset of the CDMA signal, said instructions being
arranged, upon execution thereof, to cause one or more processors
to perform the steps of the method according to claim 1.
9. A digital signal processing system including a digital signal
processor and a memory and configured to perform the steps of the
method according to claim 1.
10. A method for determining a time offset incurred by a CDMA
signal during transfer, said method comprising: receiving the CDMA
signal, said CDMA signal borne on a plurality of data channels and
a pilot channel; estimating a plurality of amplification factors
for a plurality of corresponding time offset estimates; and
determining the time offset based on a maximum of the amplification
factors for the time offset estimates when a selected data channel
is active or based on a minimum of the amplification factors for
the time offset estimates when the selected data channel is not
active.
11. The method according to claim 10, wherein: each of said data
channels is spread with a corresponding spreading code to a chip
sequence and subjected to amplification factors; a combined CDMA
signal from the data channels and the pilot channel is multiplied
by a scrambling code; and the step of estimating includes:
multiplying the chip sequence with a conjugate complex scrambling
code that is shifted by a number of chips according a time offset
estimate; despreading the chip sequence based on the spreading code
of at least one of the data channels to produce a symbol sequence;
and summing an amount of the symbol sequence to determine an
estimated amplification factor.
12. A computer-readable medium bearing instructions for determining
the time offset of the CDMA signal, said instructions being
arranged, upon execution thereof, to cause one or more processors
to perform the steps of the method according to claim 10.
13. A digital signal processing system including a digital signal
processor and a memory and configured to perform the steps of the
method according to claim 10.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for determining
the time offset of a Code Division Multiple Access (CDMA) signal,
in particular for third generation mobile radio devices.
BACKGROUND OF THE INVENTION
[0002] The channel structure of a mobile radio system for third
generation of mobile phones is described for instance in the book
by T. Ojanper, R. Prasad: "Wide Band CDMA for Third Generation
Mobil Communication", Artech House Publishers, 1998, ISBN
0-89006-735-X, in the chapter "6.3.3 Physical Channels" (pp.
169-173). It is described therein that several data channels,
referred to as Dedicated Physical Data Channels (DPDCH), and one
control or pilot channel, referred to as a Dedicated Physical
Control Channel (DPCCH), are available. While the data channels
DPDCH are available for transfer of user data, the pilot channel
DPCCH is used to transfer a pilot signal. It is proposed in the
chapter "5.5.3 Pilot Signals" that pilot signal symbols of the
pilot channel DPCCH be used exclusively for synchronization.
[0003] However, a disadvantage in using the pilot signal DPCCH for
synchronization, which was commonly employed up until now, is that
the relative output component of the pilot channel DPCCH is reduced
with an increasing activity of the data channel DPDCH, making it
difficult to achieve a reliable detection of the correlation
peak.
[0004] Therefore, there is a need for a reliable determination of
the time offset of the CDMA signal.
SUMMARY OF THE INVENTION
[0005] The present invention stems from the realization that it is
advantageous to use one or more channels (for example, a data
channel DPDCH) in the transfer path other than the pilot channel
DPCCH for the determination of the time offset. Accordingly, the
reception signal is first descrambled, wherein the conjugated
complex scrambling code is shifted by a certain number of chips.
This number of shifted chips belongs to one of several time offset
estimates to be tested. After that, the symbol sequence is again
established through despreading by using the spreading code of the
selected data channel and finally, an estimated amplification
factor is determined by adding up the amounts of this despreaded
symbol sequence. This procedure is repeated for all possible time
offsets within a certain range, preferably within the scrambling
range. If it is known that the selected data channel is active,
that is to say that activity involving data signal is taking place
therein, then the object is to determine the maximum of the
amplification signal. On the other hand, if it is known that the
selected channel is inactive, that is to say that activity
involving data signal is not taking place therein, then the minimum
amplification factor needs to be determined instead. The respective
time offset estimate in which the maximum or the minimum of the
amplification factor occurs is the correct estimate, so that the
proper number of offset chips will correspond to the time offset of
the CDMA signal to be determined.
[0006] The advantage of the method according to the invention is in
particular due to the fact that activity in several data channels
at the maximum or minimum peak of the amplification factor is more
distinct that the maximum or minimum peak of the pilot channel. In
addition to the smaller output allocation of the pilot channel when
compared to the data channels, the advantage according to this
invention is also achieved because the pilot channel of customary
mobile radio systems uses a very large spreading factor (for
example SF=256). For the data channels can be used for example a
3GPP uplink in such a way so that a very high spreading factor is
used at the beginning for the first data channel and the spreading
factor is then successively reduced when higher data rates are
desirable for transfer. If the data rate is still not sufficient
even when the smallest spreading factor is used (for example SF=4),
then a second data channel will be also used for operations with
the smallest spreading factor, etc. The result of this method is
that either at least one data channel that does not have any
activity yet is constantly available, wherein smaller outputs of
high code categories will be also copied in the code category of
the smallest spreading factor, or at least one data channel will be
available in which the code category is occupied with the smallest
spreading factor activity. Detection can thus be performed
constantly with the smallest spreading factor, that is to say with
the lowest code category, which means that the length of the
observation and the time required for processing are significantly
reduced.
[0007] When it is not known whether the selected data channel is
active or not, then both the maximum as well as the minimum of the
amplification factor will be determined. If a distinct maximum is
apparent, then the data channel will be determined as active. If a
distinct minimum is apparent, then the data channel will be
determined as not active.
[0008] The received chip sequence is preferably evaluated in a time
window which corresponds to an integral multiple of the length of
the spreading code.
[0009] A further acceleration of the method according to the
invention can be achieved wherein all possible permutation of the
product of the conjugate complex of the scrambling code with the
spreading code which can occur with despreading are deposited per
each symbol in a permutation matrix.
[0010] Without this measure, a plurality of the same
multiplications would have to be performed during an evaluation of
the different time offset estimate. The expense can be reduced by
means of the permutation matrix because a concrete permutation of
the permutation matrix can be accessed for a concrete time offset
estimate for each symbol. At the same time, there are many
4.sup.SF-1 permutations, where SF is the spreading factor. If a
data channel is used for the evaluation, which is either not active
or which is active with the smallest spreading factor SF=4, then
this will result in a calculation expense which is still executable
thanks to a relatively small number of the permutations. The table
method is effective with a small spreading factor. It is also
possible, for example when spreading factor is 8, to use the tables
of spreading factor of 4 with a supplementary addition to be used
as well (a mixed form consisting of the table method and
calculation).
[0011] Still other aspects, features, and advantages of the present
invention are readily apparent from the following detailed
description, simply by illustrating a number of particular
embodiments and implementations, including the best mode
contemplated for carrying out the present invention. The present
invention is also capable of other and different embodiments, and
its several details can be modified in various obvious respects,
all without departing from the spirit and scope of the present
invention. Accordingly, the drawing and description are to be
regarded as illustrative in nature, and not as restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The present invention is illustrated by way of example, and
not by way of limitation, in the figures of the accompanying
drawings and in which like reference numerals refer to similar
elements and in which:
[0013] FIG. 1 shows a channel and transfer model based on the
method according to the invention;
[0014] FIG. 2 is a block diagram of a first embodiment of the
method according to the invention;
[0015] FIG. 3 illustrates the amplification factors in an example
in which the first data channel DPDCH.sub.1 is active and the other
data channels are inactive;
[0016] FIG. 4 illustrates the amplification factors in an example
in which the first data channel DPDCH.sub.1 and the second data
channel DPDCH is active and the remaining data channels are
inactive;
[0017] FIG. 5 illustrates the amplification factors in an example
in which the first through the fifth data channels DPDCH.sub.1
through DPDCH.sub.5 are active and the sixth data channel
DPDCH.sub.6 is inactive, and
[0018] FIG. 6 is a block diagram of a second embodiment of the
method according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] A system, method, and software for determining the time
offset of a CDMA signal are described by way of an example using a
3GPP uplink; the present invention, however, is not so limited and
capable of general applicability. In the following description, for
the purposes of explanation, numerous specific details are set
forth in order to provide a thorough understanding of the present
invention. It is apparent, however, to one skilled in the art that
the present invention may be practiced without these specific
details or with an equivalent arrangement. In other instances,
well-known structures and devices are shown in block diagram form
in order to avoid unnecessarily obscuring the present
invention.
[0020] FIG. 1 shows a channel and a transfer model based on the
method according to this invention. Several data signal symbols
a.sub.1(.nu.) through a.sub.6(.nu.) are transmitted by a sender 1
by data channels DPDCH.sub.1 through DPDCH.sub.6 with different
spreading codes sp.sub.1(k) through sp.sub.6(k). At the same time,
pilot signal symbols a.sub.C(.nu.) of a pilot channel DPCCH are
transmitted with a spreading factor sp.sub.C(k). The symbol
sequence indicated with the index .nu. is at that point spread in a
chip sequence indicated with the index k. To the pilot channel
DPCCH or to each pilot channel DPCCH is assigned an amplification
factor g.sub.C or g.sub.1 through g.sub.6. The amplification factor
is zero when the corresponding channel is not active. The
amplification factors of active channels can have different values.
Usually, the amplification factor g.sub.C of the pilot channel
DPCCH is smaller that the amplification factor g.sub.1 through
g.sub.6 of the data channels DPDCH. The spreading with the
spreading factors sp.sub.1 through sp.sub.6 or sp.sub.C is
performed with multipliers 2.sub.1 through 2.sub.6 or 2.sub.C. The
multiplication with the amplification factors g.sub.1 through
g.sub.6 or g.sub.C is performed with multipliers 3.sub.1 through
3.sub.6 or 3.sub.C.
[0021] While the data channels DPDCH.sub.1, DPDCH.sub.3 and
DPDCH.sub.5 are integrated with the I (in-phase) components of the
complex data signal in a summer 4, the digital data channels
DPDCH.sub.2, DPDCH.sub.4 and DPDCH.sub.6 as well as the pilot
channel DPCCH are integrated in a summer 5 with the Q (quadrature)
components of the complex data signal. In another multiplier 6 is
then performed the multiplication with the scrambling code
sc(k).
[0022] After that will occur transfer in the transfer channel 7,
for instance a radio distance of a mobile radio system. The signal
will undergo in the transfer channel 7 a time offset corresponding
to a certain number .alpha. of chips. This can be modeled with a
delay element 8. From the chip sequence s(k) will thus be obtained
at the output of the sender 1 the chip sequence s(k-.alpha.). After
that will occur a frequency shift .alpha..omega. and a phase shift
.phi., which will apply a multiplier 9 to the signal in the model.
In an adder 10 is then performed in the model also the addition of
a white noise signal w(k), so that at the end of the transfer
channel 7 will now be available the chip sequence r(k) to be
received.
[0023] FIG. 2 shows a block diagram of the First Embodiment
according to the method of this invention. The method serves in
accordance with the invention to determine the time shift of CDMA
signal, that is to say the number .alpha. of chips by which the
CDMA signal s(k) is offset during the transfer in the transfer
channel 7. The chip sequence r(k) is evaluated within a time window
11. Preferably, the length of the time window 11 corresponds to an
integral multiple of the length of the spreading factor, that is to
say in the time window 11 is contained a number Anz.sub.chips of
chips that corresponds to a multiple of the length of the spreading
code sp(k). The received chip sequence r(k) is multiplied in a
multiplier 12 with a window function which is within the time
window 1 and outside of the time windows 0. The received chip
sequence is then multiplied in a multiplier 13 with the conjugate
complex offset with scrambling factor sc*(k-{tilde over
(.alpha.)}). Various corresponding time offset estimates are then
formed, which are different with respect to the various estimates
applied to the time offset, that is to say the number {tilde over
(.alpha.)} of chips by which the signal is offset during the
transfer in accordance with the estimate. Because of that, the
scrambling code must be offset in accordance with the time offset
on which the estimate is based. The received chip series r(k) is
calculated in a despreading block 14 according to the Formula: 1 r
v , i ( ~ ) = K = K v K v + SF - 1 r ( k ) sc * ( k - ~ ) sp i ( k
) ( 1 )
[0024] The symbols in the formula indicate the following:
[0025] k chip index
[0026] .nu. symbol index
[0027] i index of the data channel DPDCH,
[0028] k.sub..nu. the beginning of the .nu.-th symbol
[0029] r(k) the received chip sequence
[0030] sc*(k-{tilde over (.alpha.)}) the conjugate complex
spreading code offset by {tilde over (.alpha.)}.sub.chips
[0031] sp.sub.i(k) the spreading code of the i-th data channel
DPDCH.sub.1
[0032] SF the spreading factor, that is to say the length of the
spreading code sp.sub.i(k)
[0033] {tilde over (.alpha.)} the number of the chips of the time
offset on which the estimate is based.
[0034] The evaluation is then performed for a selected data channel
DPDCH.sub.i, specifically by utilizing the spreading code
sp.sub.i(k). It is preferable when a data channel is used whose
spreading code sp.sub.i(k) is relatively short.
[0035] The amounts of the despreading symbols r.sub..nu.,i({tilde
over (.alpha.)}) which has a complex value are then calculated in
an amount component forming unit 15 and the mean value of the
amounts of the despreading symbol sequence r.sub..nu.,i({tilde over
(.alpha.)}) is calculated in a mean value calculation device 16.
The mean value of the amounts of the despreading sequence of
symbols is the current estimated amplification factor .sub.i({tilde
over (.alpha.)}) for the i-th data channel DPDCH.sub.i based on the
estimate that a time offset by {tilde over (.alpha.)} chips occurs
in the transfer channel 7. The division by the number of the
symbols Anz.sub.sym can be naturally also omitted in order to
simplify implementation.
[0036] The above described method steps are repeated for all
possible time offset estimates, that is to say for all possible
values from {tilde over (.alpha.)}, with 0.ltoreq.{tilde over
(.alpha.)}<38400.
[0037] If it is known that selected data channel DPDCH.sub.i is an
active data channel, then the maximum max of all amplification
factors .sub.i({tilde over (.alpha.)}) will be sought in the
maximum/minimum evaluation unit 17. On the other hand, if it is
known that that the channel DPDCH.sub.i which is used for
evaluation is inactive, then the minimum min will be sought in the
maximum/minimum evaluation unit 17 for all amplification factors
.sub.i({tilde over (.alpha.)}). In addition, the mean value mean or
the median value median can be also calculated. If it is not known
whether the data channel is active, the both the maximum and the
minimum of the amplification factors will be sought. If the
evaluated data channel DPDCH.sub.i is active, then the time offset
estimate a in which a maximum of the amplification factor
.sub.i({tilde over (.alpha.)}) occurs will be decided upon. The
{circumflex over (.alpha.)} belonging to this maximum or minimum
represents the time offset of the received CDMA signal r(k) to be
determined for the sent CDMA signal s(k) in the unit of chip
periods.
[0038] It is generally also possible to use multiple data channels
which should be preferably all active or passive.
[0039] The maximum or minimum is then determined from a linear
combination of the amplification factor .sub.i({tilde over
(.alpha.)}).
[0040] The result of the method according to the invention and its
advantages in comparison to evaluation of the pilot channel DPCCH
will now be explained with reference to FIGS. 3 through 5.
[0041] In the example which is illustrated in FIG. 3, the first
data channel DPDCH.sub.i is active in addition to the pilot channel
DPCCH, while the other channels DPDCH.sub.2 through DPDCH.sub.6 are
inactive. The observation length comprises a half slot=1280 chips.
The figure represents the amplification factor .sub.i({tilde over
(.alpha.)}) established in accordance with the help of the above
explained method depending on the respective time offset estimate
based on the value {tilde over (.alpha.)}, wherein only the
estimate {circumflex over (.alpha.)} of 200 chips are taken into
account. In the first line is outlined the amplification factor
.sub.c({tilde over (.alpha.)}) for the pilot channel DPCCH. In the
second line is outlined a combined amplification factor .sub.12
({tilde over (.alpha.)}) for both data channels DPDCH.sub.1 and
DPDCH.sub.2. One should note here that the spreading codes
sp.sub.1(k) and sp.sub.2(k) differ only by a factor j, that is to
say the odd numbered data channels modulate the I components and
the even numbered data channels modulate the Q components. Since
according to the method of this invention, the amounts of the data
symbol r.sub..nu.,i({tilde over (.alpha.)}) are evaluated, the
phase can be ignored during the determination of the amplification
factor, so that the first and the second data channels can not
differ from each other. In the third line is outlined the
amplification factor .sub.34 ({tilde over (.alpha.)}), wherein also
here according to the method of this invention, the same result
will be provided for the third data channel DPDCH.sub.3 and for the
fourth data channel DPDCH.sub.4. Accordingly, in the fourth line is
illustrated the determined amplification factor .sub.56({tilde over
(.alpha.)}) wherein the data channels DPDCH.sub.5 and DPDCH.sub.6
have the same result. The example illustrated in FIGS. 3 through 6
is based on the assumption that no time offset is produced, that is
to say that the equation .alpha.=0 is valid.
[0042] With the data channel DPDCH.sub.1 that was the only active
one in FIG. 3 will be obtained another information-intensive peak
(maximum value) of the amplification factor .sub.C({tilde over
(.alpha.)}) also for pilot channel DPCCH. The maximum value of the
amplification factor .sub.1({tilde over (.alpha.)}) of the data
channel DPDCH.sub.1 can be, however, better evaluated based on the
larger output allocation for the data channel and on the basis of
the fact that for the pilot channel DPCCH is used the largest
spreading factor, while a smaller spreading factor is used for the
data channel DPDCH.sub.1 in the example. Clearly distinguishable
are the minimum values of the amplification factors .sub.34({tilde
over (.alpha.)}) and .sub.56({tilde over (.alpha.)}) of the
non-active data channels DPDCH.sub.3 through DPDCH.sub.6, which can
be very easily evaluated.
[0043] In FIG. 4 are shown the same parameters as in FIG. 3. The
basic example differs here however in that in addition to the first
data channel DPDCH.sub.1, a second data channel DPDCH.sub.2 is also
active, while the third through the sixth channel (DPDCH.sub.3
through DPDCH.sub.6) continue to be inactive. A clear difference in
comparison to FIG. 3 is that the peak of the amplification factor
.sub.C({tilde over (.alpha.)}) of the pilot channel DPCCH has
clearly weaker characteristics due to the relatively smaller output
component of this channel when compared to FIG. 3, so that a safe
evaluation is now barely possible. In contrast to that, the maximum
of the amplification factor .sub.12({tilde over (.alpha.)})
continues to have clearly strong characteristics. Also the minimum
values of the amplification factors .sub.34({tilde over (.alpha.)})
and .sub.56({tilde over (.alpha.)}) can be detected without any
problem.
[0044] The illustration of the parameters in FIG. 5 is again
similarly to the illustration in FIG. 4 and FIG. 5 based on an
example wherein in addition to the pilot channel DPCCH, data
channels DPDCH.sub.1, DPDCH.sub.2, DPDCH.sub.3, DPDCH.sub.4 and
DPDCH.sub.5 are also active, while only the data channel
DPDCH.sub.6 is inactive. A peak can no longer be recognized in this
example for the amplification factor .sub.C({tilde over (.alpha.)})
with {tilde over (.alpha.)}=0. On the other hand, the maximum of
the amplification factor .sub.12({tilde over (.alpha.)}) as well as
of the amplification factors .sub.34({tilde over (.alpha.)}) and
.sub.56({tilde over (.alpha.)}) is clearly recognizable and can be
easily evaluated.
[0045] It is however also possible to use a channel that was never
active, for example with SF=8, in addition to the code tree of
DPCCH (not shown).
[0046] The advantage of the improved evaluation capability of the
amplification factors of the Data channels DPDCH in accordance with
this invention in comparison to the pilot channel is due to the
fact that the pilot channel DPCCH of most mobile radio systems, for
instance during an uplink (connection from a mobile station to a
base station) is constantly assigned a higher spreading factor SF,
for example SF=256. The first data channel DPDCH.sub.1 has a
spreading factor 4<=SF<=256, wherein SF=4 with the 3GPP
downlink of the smallest spreading factor. If a higher data rate is
not required, the data channels DPDCH.sub.2-DPDCH.sub.6 are
inactive and the amplification factors .sub.34({tilde over
(.alpha.)}) and .sub.56({tilde over (.alpha.)}) can be evaluated
with respect to their minimum values without any problem (the
example of the case shown in FIG. 3). If a higher data rate is
required, then in case of the 3GPP uplink, first the spreading
factor of the data channel DPDCH.sub.1 will be reduced in stages
until SF=4 is reached for DPDCH.sub.1. If a still higher data rate
is required, then the other data channels are connected
successively with a spreading factor SF=4. In the despreading block
14 can thus be always used spreading codes sp.sub.i(k) with SF=4
either for detection of the maximum of an active data channel, or
for detection of the minimum of an inactive data channel. The above
described circumstance facilitates the evaluation considerably,
because during the evaluation of the pilot channel DPCCH customary
up until now, in the despreading block 14 had to be used a
spreading factor of SF=256, which would require a significantly
higher calculation expense and a higher observation time than the
evaluation of the data channel DPDCH according to the invention
with a spreading factor of SF=4.
[0047] A block diagram showing a second embodiment of the method of
this invention will now be explained with reference to FIG. 6. The
elements that were already described in FIG. 2 are provided with
the same reference symbols so as to render repeated description
unnecessary.
[0048] The further development of the invention shown in FIG. 6 is
based on the realization that formula for despreading 2 r v , i ( ~
) = K = K v K v + SF - 1 r ( k ) sc * ( k - ~ ) sp i ( k ) ( 1
)
[0049] with the factor
sc*(k-{tilde over (.alpha.)}).multidot.sp.sub.i(k)
[0050] only 4.sup.SF permutations can be assumed, wherein SF is the
spreading factor of the spreading code sp.sub.i(k). Since the
received chip sequence r(k) is identical for each amplification
factor .sub.i({tilde over (.alpha.)}) to be calculated, that is to
say for each value {tilde over (.alpha.)}, this means that a
plurality of multiplications must be calculated several times in
the summands of the formula above. In order to reduce the
calculating time, it is therefore advantageous to calculate all the
permutation of the term
sc*(k-{tilde over (.alpha.)}).multidot.sp.sub.i(k)
[0051] and to calculate all the 4.sup.SF-1 permutations for a
received chip sequence r(k) for symbol r.sub..nu.,i({tilde over
(.alpha.)}) to be calculated for each symbol in advance only once
and to deposit these permutations for each symbol in a permutation
matrix. In order to illustrate clearly this principle, FIG. 6
indicates a permutation table 20 which contains four complex value
permutations of the term
sc*(k-{tilde over (.alpha.)}).multidot.sp.sub.i(k)
[0052] In this case the factor sc*(k-{tilde over (.alpha.)})
assumes only the values +-1+-j, while for the factor sp.sub.i(k)
can be assumed only the values +-1. The amounts of the symbols
r.sub..nu.,i({tilde over (.alpha.)}) calculated for all the
permutations are stored in intermediate storage for all symbols and
for all permutations in a permutation matrix 21. With summation via
the SF summands, there will thus be 4.sup.SF permutations for each
symbols r.sub..nu.,i({tilde over (.alpha.)}). During the
calculation of the amplification factor .sub.i({tilde over
(.alpha.)}), the present {circumflex over (.alpha.)} will be
accessed for a concrete present permutation for each symbol
r.sub..nu.,i({tilde over (.alpha.)}) by of an address generator
22.
[0053] The calculation time can be clearly reduced with the
permutation matrix 21. For a spreading factor SF=4 will thus be
produced 4.sup.SF-1=64 permutations for each symbol. With a
scrambling range consisting of 38400 chips and an observation
length of 2560 chips, the calculation expense will be reduced when
compared to a linear implementation approximately by a factor of
150.
[0054] Accordingly, a quick determination of the time offset of
received CDMA signal is enabled.
[0055] Embodiments of the present invention can be implemented on
computer-readable media bearing instructions that are arranged,
when executed, to cause one or more computer or digital signal
processors in a digital processing system. to determine the time
offset of a CDMA signal. The term "computer-readable medium" as
used herein refers to any medium that participates in providing
instructions to a computer or digital signal processor for
execution. Such a medium any take many forms, including but not
limited to non-volatile media, volatile media, and transmission
media. Non-volatile media include, for example, optical or magnetic
disks. Volatile media include dynamic memory. Transmission media
include coaxial cables, copper wire and fiber optics, including the
wires that comprise a computer bus. Transmission media can also
take the form of acoustic, optical, or electromagnetic waves, such
as those generated during radio frequency (RF) and infrared (IR)
data communications. Common forms of computer-readable media
include, for example, a floppy disk, a flexible disk, hard disk,
magnetic tape, any other magnetic medium, a CD-ROM, CDRW, DVD, any
other optical medium, punch cards, paper tape, optical mark sheets,
any other physical medium with patterns of holes or other optically
recognizable indicia, a RAM, a PROM, and EPROM, a FLASH-EPROM, any
other memory chip or cartridge, a carrier wave, or any other medium
from which a computer can read.
[0056] While the present invention has been described in connection
with a number of embodiments and implementations, the present
invention is not so limited but covers various obvious
modifications and equivalent arrangements, which fall within the
purview of the appended claims.
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